Reporter proteins have made an immeasurable contribution to the advancement of life sciences. The most commonly used reporter protein is β-galactosidase. A relationship between aging and expression of β-galactosidase in cells has recently been suggested (see Non-Patent Reference 1), and imaging probes which are enzyme-specific to β-galactosidase are important molecular tools for elucidating mechanisms of cell aging. Furthermore, β-galactosidase activity has been shown to be elevated in certain types of cancer cells (see Non-Patent References 2 and 3), and an imaging probe which is enzyme-specific to β-galactosidase is thought to be usable also as a cancer-cell-selective fluorescence imaging probe.
Conventional methods for imaging enzyme activity using X-Gal as a substrate are widely used (Non-Patent Reference 4), but X-Gal cannot be applied to living cells, and there is therefore a need to develop a probe for imaging enzyme activity which can be applied to living cells. Numerous imaging probes that can be applied to living cells have presently been developed. For example, HMDER-βGal and the like have been developed as β-galactosidase fluorescent probes which can be applied to living cells and living biological tissues, and in which visible light excitation is possible by control of a spiro-ring-forming reaction in the molecule thereof (see Non-Patent Reference 5 and Patent Reference 1). However, in all of these fluorescent probes, the enzyme reaction product leaks from the cells, or cytotoxic ultraviolet light is used as excitation light, and living cells and the like are difficult to clearly image at a single-cell level. Conventional cancer probes also have drawbacks in that immobilization of a section for pathological diagnosis causes the cancer probe to leak out of cells, making the cancer probe unusable for pathological diagnosis.